1. Field of the Invention
The present invention relates to a recording medium and an image recording set.
2. Background Art
In recent years, electronic paper has been put into practical use and gained attention because it is visible and portable like paper material, thin, and lightweight requiring no backlight and less power consumption.
Electronic paper is classified into internally-driven paper-like display (hereinafter referred to as PLD) that includes a drive element inside to display images by itself and externally-driven rewritable paper using a recording medium on which writing is performed by a printer.
The rewritable paper includes all recording media such as paper and sheets on which rewriting can be repeatedly performed.
Various systems of electronic paper have been proposed. Of these, as a PLD, an electrophoretic system, that conducts display by changing an optical reflectance by migrating white and/or colored particles charged in a liquid to the surface side or the opposite direction of a display medium by an electric field is known as the most practical system.
As an example of the electronic paper of such system, JP-S50-15115-B discloses a display or recording device, in which an electric field is applied to a mixed multi particle dispersion system containing particles that migrate in opposite directions depending on the electric field, to optically shield certain particles, thereby changing optical reflection characteristics of the dispersion system. In addition, JP-2551783-B1 (JP-H01-086116-A) discloses an electrophoretic display device, in which a large number of microcapsules each encapsulating a dispersion system obtained by dispersing electrophoretic particles in a colored dispersion medium are provided between a pair of counter electrode plates.
Furthermore, JP-4410135-B1 (JP-2005-227795-A) discloses an ink, in which an electric field is applied to microcapsules each containing first particles and second particles having reverse polarity to that of the first particles to thereby migrate either the first particles or second particles to the surface.
As described above, the electrophoretic system, in particular a microcapsule electrophoretic system, has already been put into practical use as electronic paper and become appealing.
Such electrophoretic electronic paper is capable of retaining displayed content without requiring electric power once displaying the content, which is recognized as one of the advantages. However, in fact, image density changes with time unless an electric field is applied or image quality deteriorates by static electricity and pressure from outside.
In attempts to solve these problems, for example, JP-2007-286124-A discloses a method of controlling a solubility parameter in a dispersion medium by making walls of microcapsules formed by a graft copolymer. Also, JP-2010-002933-A discloses a method of dissolving or dispersing a polymer that does not become thick in a fluid suspension.
These technologies are appropriate to improve the image retention (retentiveness) required for PLD.
However, these are not sufficient to retain images on rewritable paper.
This difference lies in the structures and the purposes of use of PLD and rewritable paper.
PLD has a substrates each having an electric conductive layer, as electrodes, at the top and the bottom of a display layer in which electrophoretic particles move and is covered by a housing, so that it is never or little affected from the outside. In addition, a voltage can be applied thereto if desired, thereby retaining an image on PLD.
However, rewritable paper is not covered by a housing and is easily affected from the outside. In addition, a recording medium includes no element or power source to apply a voltage, meaning that retaining an image from outside for a long period of time is impossible.
It is possible to reduce an impact from outside by providing a protective layer to cover rewritable paper in some cases. However, this is not sufficient to enhance image retention. As a result, advantages such as flexibility and low cost of rewritable paper are lost.
Taking into account the purpose of PLD, images are frequently rewritten and demand for image retention is not strict. By contrast, rewritable paper is required to retain records for a long period of time, almost permanently, like paper.
As a technology to significantly enhance image retention, there are a number of methods including controlling the viscosity of a dispersion medium in which electrophoretic particles are dispersed, lowering the viscosity by heating to conduct recording, and thereafter fixing colored particles by cooling-down.
For example, JP-S50-15115-B1 mentioned above discloses a method of using a resin, rubber, waxes, or synthetic waxes that are solid at normal temperatures and softened thermally or chemically.
JP-4168556-B1 (JP-2001-147451-A) discloses a method of forming a uniform liquid phase by dissolving a solid phase of a dispersion medium, which has both solid and liquid phases present at room temperature, during electrophoretic migration in a liquid dispersion containing electrophoretic particles, the dispersion medium, a dye, and a dispersant.
JP-2001-301325-A discloses a method in which a heat-meltable substance that is solid when it is not heated and becomes liquid when it is heated, specifically, waxes, saturated fatty acid, higher alcohols, and electrophoretic fine particles are encapsulated into microcapsules.
JP-2002-365670-A discloses a method using a mixture that has fluidity at an electrophoretic operating temperature, and becomes an optically opaque solid at temperatures lower than the electrophoretic operating temperature, specifically, a mixture of a substance selected among long-chain alcohols, phenols, aromatic ketones, decanes, and dodecanes and a substance selected among paraffin, waxes, and saturated hydrocarbons.
JP-2003-91022-A discloses a method using higher paraffin hydrocarbons, waxes, or aromatic hydrocarbons as dispersion medium that is solid at normal temperatures and becomes liquid by heating.
JP-2007-140367-A discloses a method in which a dispersion medium is composed mostly of rice wax and contains a surfactant.
Thus, a dispersion medium that is solid at normal temperatures and becomes liquid when being heated is very effective for improvement in image retention because electrophoretic particles are fixed by cooling after recording.
However, since these technologies use a dispersion medium to which waxes and saturated aliphatic acids are added, temperature responsiveness is low in transition between the solid state and the liquid state of electrophoretic particles, which causes problems such that rewriting speed decreases, the viscosity of a dispersion medium is not sufficiently lowered even when heated, thereby degrading contrast, or the quality of an image deteriorates over repeated rewriting.
Further, the dispersibility of electrophoretic particles significantly deteriorates when compared to a case where a solvent is used as the dispersion medium. For this reason, electrophoretic properties may be impaired or particles aggregates, thereby degrading image quality.
A method of using a dispersion medium that turns into a gel in a PLD is disclosed. For example, JP-H02-196227-A discloses a thermal writing system which includes a dispersion medium that turns into a gel and a sol (liquid) reversibly depending on temperatures and electrophoretic particles and locally migrates the electrophoretic particles by locally converting the dispersion medium into a sol.
However, with this technology, as a dispersion medium that turns into a gel and a liquid, specifically, a solvent is used for a high-molecular weight substance, resulting in a poor solvent. This high-molecular weight substance is dissolved when the temperature of the solvent is high and the high-molecular substance becomes insoluble and turns into a gel when the temperature is low, so that the high-molecular substance itself does not have thermal reversibility.
Therefore, the temperature responsiveness is low in transition between a gel and a liquid and liquefaction is locally limited, so that display speed and image quality are not satisfactory.
JP-2003-149691-A discloses a display gel for electrophoretic display containing a dispersion medium, white particles, colored particles, and a gelling agent.
However, in this technology, although a gelling agent is contained in the dispersion medium, thermal reversibility is not described. Actually, due to gellation, the dispersion medium is substantially equally divided into continuous and/or discontinuous minute regions and turns into a gel and the particles electrophoretically migrate by the action of an electric field in the dispersion medium in a gel state.
Therefore, the problems regarding rewriting speed and contrast remain unsolved.
JP-2007-11342-A discloses a display medium for electrophoretic display containing a set of a liquid system, a thermoreversible gelling agent, and colored particles for PLD.
With this technology, by containing the thermoreversible gelling agent, the display medium turns into non-gel and is reduced in viscosity at temperature close to or higher than the melting point of the gelling agent, so that images can be formed in a display device. When the display device is cooled after an image is formed, the medium turns into a gel, so that the image is stably maintained without an application of an electric field.
In the method using a thermoreversible gelling agent, the gelling agent itself causes gelatinization, so that the temperature responsiveness is higher and more stable than in the conventional technologies in which waxes and solders are added or gelatinization is controlled by the solubility of a high-molecular substance in a solvent, so that this method is very excellent for enhancing image retentiveness.
However, considering that the thermoreversible gelling agent is also controlled by temperature, the adverse impact on reduction in rewriting speed becomes large for frequent image rewriting.
As described above, since this technology using a thermoreversible gelling agent is used for a display device, that is, a PLD, deterioration of display responsiveness is greater than the advantage of semipermanent image retentiveness demonstrated by using a thermoreversible gelling agent.
In addition, a heating device and a cooling device must be provided inside the device, so that the thickness and weight of the PLD increase, which also creates a large disadvantage. Therefore, the effect of use of the thermoreversible gelling agent has not been sufficiently utilized nor applied to a PLD in reality.
The PLD, mainly used as a conventional display, has currently been largely developed and significantly advanced. However, the advancement of the rewritable paper, which is mainly used as conventional paper, has been slightly delayed as compared with the PLD.
However, there is a need for paper different from that for a display, and the spread of rewritable paper that can be handled in the same manner as paper has been expected.
Problems of rewritable paper lie in image retentiveness, image quality, and rewriting property.
As described above, age retentiveness is a large issue for rewritable paper rather than for PLD.
The PLD is configured as a device for rewriting, so that the frequency of rewriting is naturally high.
Therefore, it does not necessarily require semipermanent image retentiveness.
On the other hand, although the rewritable paper is rewritable, high image retentiveness is required considering that it is used instead of paper.
In addition, since rewritable paper is not covered by a housing, it is more vulnerable to external static electricity and pressures than the PLD, which is covered by a housing, so that enhancing image retentiveness is a big problem for rewritable paper.
Therefore, image retentiveness is a very significant issue for rewritable paper rather than for PLD.
For this reason, a thermoreversible gelling agent is more useful for rewritable paper than PLD.
However, the image quality of rewritable paper is significantly dependent on the kind of a thermoreversible gelling agent so that it is necessary to appropriately select a thermoreversible gelling agent to improve the image quality.
For example, a dispersion medium containing a thermoreversible gelling agent precipitates during gelation unless an appropriate gelling agent is used. Such precipitation leads to production of defective images with beads, which results in low contrast and uneven image density.
In addition, such a dispersion medium has come to be crystallized while repeating solation and gelation, so that the dispersing agent is not solated in the end, which makes it impossible to rewrite images.